Computer graphics systems are now available which allow the researcher to study or view various types of data as three dimensional images on a display screen. Of the three types of graphics systems available, namely, storage tube, calligraphic and raster, the present invention finds particular use in the latter. In raster graphics systems, the display screen can be thought of as an XY grid pattern. Each discrete cell or element in the grid pattern is referred to as a pixel, i.e. picture element. In raster graphics systems, each pixel can be displayed in a desired manner, i.e. brightness, color, etc. See Rogers, D. F. Procedural Elements for Computer Graphics. N.Y., McGraw-Hill, 1985, pp. 3-15. Due to such versatility, raster graphics systems have become quite popular.
In general, a raster graphics system includes an image creation section, an image storage section, an image display section and a raster display which can be of several types, including a cathode ray tube (CRT). In such a raster graphics system, the image creation section converts signals generated by one or more applications (computer programs) into pixel information which is stored in a frame buffer. Information relating to each pixel is written to a particular memory location in the frame buffer for eventual screen display. Such memory locations are referred to as "on screen" memory, which typically forms only a part of the overall frame buffer memory. Memory locations to which pixel information is not written are referred to as "off screen" memory. Pixel information is read from the frame buffer and provided to the image display section where the digital data is converted to one or more analog signals. The analog signals are designed to achieve the desired pixel image when applied to the raster display.
It will be understood that if the raster display is a CRT, the image displayed on the CRT must be updated (refreshed) at least 30 times per second in order for the eye to perceive a continuous picture. Consequently, the time required to access pixel information stored in the on screen portion of the frame buffer is critical to raster graphics systems incorporating CRT displays. For example, in the case of a 512.times.512 raster display, if pixels are accessed individually with an average access time of 200 nanoseconds, then it requires 0.0524 seconds to access each pixel from a 512.times.512 on screen memory. This is equivalent to a refresh or frame rate of approximately 19 frames per second, well below the required minimum refresh rate of 30 frames per second. The average access time must be quicker. As the number of pixels increases, i.e. increased resolution, the rate at which pixels are accessed becomes even more critical.
More realistically, the on screen memory portion of the frame buffer of a contemporary raster graphics system will include a twenty four bit-plane memory, where eight bit-planes are dedicated to each CRT primary color, i.e. red, green and blue. A bit-plane is a quantity of contiguous memory equal to the number of pixels in the raster display. For example, a 512.times.512 raster display will have a total of 262,144 bits. Each bit plane in such a system will include this same amount of memory.
In a twenty four bit-plane graphics system, approximately 188,743,680 bits per second must be accessed from the frame buffer to achieve a refresh rate of thirty frames per second. Since physically independent memory devices are utilized in the frame buffer for each CRT color (red, green, blue), only 62,914,560 bits need be accessed each second for any one color.
In many cases it is desired to increase the number of lines in the display as well as the number of pixels per line. If it is desired to increase the 512.times.512 raster display to a 1024.times.1024 raster display or greater, it will be necessary to access approximately 125,829,120 bits per second for each color.
In order to handle such large access operations particular memory devices and techniques have been utilized, for example memory devices in which eight bits of information can be retrieved at any single row and column address and the retrieval of pixel information in groups.
Since the access rate of pixel information is critical, it is important that the bandwidth of the frame buffer be maximized. As used herein, frame buffer bandwidth shall mean the number of bits per second which can be transferred from the frame buffer to the image display section of the graphics system. In other words, bandwidth equals the number of frame buffer memory locations that can be accessed per second. It will be appreciated that frame buffer bandwidth/access time will directly effect the degree of resolution which can be achieved.
U.S. Pat. No. 4,991,110--Hannah recognizes the need to increase memory bandwidth in a graphics device. The frame buffer memory is said to include a plurality of video random access memory (VRAM) devices. VRAM devices are memory devices specifically designed for the temporary data storage conditions occurring in graphics applications. Bandwidth of the random port is said to be increased by staggering the timing of row address strobe signals and column address strobe signals so that data can be transferred in a staggered fashion to each VRAM within a memory cycle. Pixel information is stored in a sequential fashion. Read operations are also said to be enhanced utilizing a staggered output enable signal.
Unfortunately, frame buffers utilizing VRAM devices exhibit bandwidth limited by the rate at which information can be shifted out of the shift register component of the VRAM devices. That is, bandwidth is limited by the number of data shifts required to access the desired pixel information stored in the VRAMs. However, VRAMs do have other desireable qualities.
Consequently, a need exists for a raster graphics system which utilize a frame buffer incorporating VRAM devices and which exhibit maximum frame buffer bandwidth.